From Fertilizer to Food: Tracing Nitrogen Dynamics in Conventional and Organic Farming Systems Using ~(15)N Natural Abundance

摘要

Synthetic nitrogen (N) fertilizers differ markedly from organic N fer-llzer sources in relative isotopic composition at natural abundance levels (615N). The objective of this paper is to provide an overview of the applications of 515N techniques to study the dynamics of synthetic fertilizers, animal excreta and composts in the soil-plant-atmosphere continuum. However, isotopic fractionation processes often complicate the interpretation of results. These fractionation processes and the factors affecting the 615N signatures of organic N fertilizers are reviewed. Published data from short-, medium- and long-term experiments with annual crop rotations and in pastures subject to organic N inputs are also examined and analyzed with respect to changes in delta nitrogen-15 (615N) signatures of the soil, the crop or pasture, the soil biota and leachates. The use of 615N to differentiate organic and conventional plant products is briefly covered. There are few data on the dynamics of N during the storage ofanimal excreta or the composting of agricultural wastes as shown by 615N values in the organic, inorganic or gaseous N phases. The major N loss process is ammonia (NH3) volatilization. Reviewed data show significant relationships between bulk 615N signatures of stored manure and cumulative NH3 loss or bulk 615N of livestock manure composts and N concentration. These significant relationships suggest that 615N may have wider applications in estimating the efficiency of N conservation during storage or composting. In addition, the combined use of bulk 515N and delta oxygen-18 (5180) signatures of nitrous oxide (N_2O) evolved during storage and composting, together with the iso-topomer-derived site preference of N20, are emerging technologies for identifying N_2O production pathways. 615N in combination with appropriate statistical analysis is a promising diagnostic tool for differentiating organic and conventional plant products.